Vlad I put a Guarder SP100 in my wife's TM MP5K and it only increased the velocity to 300 fps. Right now her gun is firing a true 360 fps but that required a PDI 170% spring in it.

It's not just the spring strength that you have to consider it's also the type, irregular springs are terrible for a short barrel weapon like this because the lighter tension parts of the spring expand first which propel the BB out of the barrel before the rest of the spring can expand.

Heh it was a pain of a learning process but the PDI 170% works well (despite what people may say about PDI's QA), regular pitch and the spring is very physically short.